RFC 1341
MIME (Multipurpose Internet Mail Extensions): Mechanisms for Specifying and Describing the Format of Internet Message Bodies
Network Working Group N. Borenstein, Bellcore
Request for Comments: 1341 N. Freed, Innosoft
June 1992
MIME (Multipurpose Internet Mail Extensions):
Mechanisms for Specifying and Describing
the Format of Internet Message Bodies
Status of this Memo
This RFC specifies an IAB standards track protocol for the
Internet community, and requests discussion and suggestions
for improvements. Please refer to the current edition of
the "IAB Official Protocol Standards" for the
standardization state and status of this protocol.
Distribution of this memo is unlimited.
Abstract
RFC 822 defines a message representation protocol which
specifies considerable detail about message headers, but
which leaves the message content, or message body, as flat
ASCII text. This document redefines the format of message
bodies to allow multi-part textual and non-textual message
bodies to be represented and exchanged without loss of
information. This is based on earlier work documented in
RFC 934 and RFC 1049, but extends and revises that work.
Because RFC 822 said so little about message bodies, this
document is largely orthogonal to (rather than a revision
of) RFC 822.
In particular, this document is designed to provide
facilities to include multiple objects in a single message,
to represent body text in character sets other than US-
ASCII, to represent formatted multi-font text messages, to
represent non-textual material such as images and audio
fragments, and generally to facilitate later extensions
defining new types of Internet mail for use by cooperating
mail agents.
This document does NOT extend Internet mail header fields to
permit anything other than US-ASCII text data. It is
recognized that such extensions are necessary, and they are
the subject of a companion document [RFC -1342].
A table of contents appears at the end of this document.
1 Introduction Since its publication in 1982, RFC 822 [RFC-822] has defined the standard format of textual mail messages on the Internet. Its success has been such that the RFC 822 format has been adopted, wholly or partially, well beyond the confines of the Internet and the Internet SMTP transport defined by RFC 821 [RFC-821]. As the format has seen wider use, a number of limitations have proven increasingly restrictive for the user community. RFC 822 was intended to specify a format for text messages. As such, non-text messages, such as multimedia messages that might include audio or images, are simply not mentioned. Even in the case of text, however, RFC 822 is inadequate for the needs of mail users whose languages require the use of character sets richer than US ASCII [US-ASCII]. Since RFC 822 does not specify mechanisms for mail containing audio, video, Asian language text, or even text in most European languages, additional specifications are needed One of the notable limitations of RFC 821/822 based mail systems is the fact that they limit the contents of electronic mail messages to relatively short lines of seven-bit ASCII. This forces users to convert any non- textual data that they may wish to send into seven-bit bytes representable as printable ASCII characters before invoking a local mail UA (User Agent, a program with which human users send and receive mail). Examples of such encodings currently used in the Internet include pure hexadecimal, uuencode, the 3-in-4 base 64 scheme specified in RFC 1113, the Andrew Toolkit Representation [ATK], and many others. The limitations of RFC 822 mail become even more apparent as gateways are designed to allow for the exchange of mail messages between RFC 822 hosts and X.400 hosts. X.400 [X400] specifies mechanisms for the inclusion of non-textual body parts within electronic mail messages. The current standards for the mapping of X.400 messages to RFC 822 messages specify that either X.400 non-textual body parts should be converted to (not encoded in) an ASCII format, or that they should be discarded, notifying the RFC 822 user that discarding has occurred. This is clearly undesirable, as information that a user may wish to receive is lost. Even though a user's UA may not have the capability of dealing with the non-textual body part, the user might have some mechanism external to the UA that can extract useful information from the body part. Moreover, it does not allow for the fact that the message may eventually be gatewayed back into an X.400 message handling system (i.e., the X.400 message is "tunneled" through Internet mail), where the non-textual information would definitely become useful again.
This document describes several mechanisms that combine to
solve most of these problems without introducing any serious
incompatibilities with the existing world of RFC 822 mail.
In particular, it describes:
1. A MIME-Version header field, which uses a version number
to declare a message to be conformant with this
specification and allows mail processing agents to
distinguish between such messages and those generated
by older or non-conformant software, which is presumed
to lack such a field.
2. A Content-Type header field, generalized from RFC 1049
[RFC-1049], which can be used to specify the type and
subtype of data in the body of a message and to fully
specify the native representation (encoding) of such
data.
2.a. A "text" Content-Type value, which can be used to
represent textual information in a number of
character sets and formatted text description
languages in a standardized manner.
2.b. A "multipart" Content-Type value, which can be
used to combine several body parts, possibly of
differing types of data, into a single message.
2.c. An "application" Content-Type value, which can be
used to transmit application data or binary data,
and hence, among other uses, to implement an
electronic mail file transfer service.
2.d. A "message" Content-Type value, for encapsulating
a mail message.
2.e An "image" Content-Type value, for transmitting
still image (picture) data.
2.f. An "audio" Content-Type value, for transmitting
audio or voice data.
2.g. A "video" Content-Type value, for transmitting
video or moving image data, possibly with audio as
part of the composite video data format.
3. A Content-Transfer-Encoding header field, which can be
used to specify an auxiliary encoding that was applied
to the data in order to allow it to pass through mail
transport mechanisms which may have data or character
set limitations.
4. Two optional header fields that can be used to further
describe the data in a message body, the Content-ID and
Content-Description header fields.
MIME has been carefully designed as an extensible mechanism,
and it is expected that the set of content-type/subtype
pairs and their associated parameters will grow
significantly with time. Several other MIME fields, notably
including character set names, are likely to have new values
defined over time. In order to ensure that the set of such
values is developed in an orderly, well-specified, and
public manner, MIME defines a registration process which
uses the Internet Assigned Numbers Authority (IANA) as a
central registry for such values. Appendix F provides
details about how IANA registration is accomplished.
Finally, to specify and promote interoperability, Appendix A
of this document provides a basic applicability statement
for a subset of the above mechanisms that defines a minimal
level of "conformance" with this document.
HISTORICAL NOTE: Several of the mechanisms described in
this document may seem somewhat strange or even baroque at
first reading. It is important to note that compatibility
with existing standards AND robustness across existing
practice were two of the highest priorities of the working
group that developed this document. In particular,
compatibility was always favored over elegance.
2 Notations, Conventions, and Generic BNF Grammar
This document is being published in two versions, one as
plain ASCII text and one as PostScript. The latter is
recommended, though the textual contents are identical. An
Andrew-format copy of this document is also available from
the first author (Borenstein).
Although the mechanisms specified in this document are all
described in prose, most are also described formally in the
modified BNF notation of RFC 822. Implementors will need to
be familiar with this notation in order to understand this
specification, and are referred to RFC 822 for a complete
explanation of the modified BNF notation.
Some of the modified BNF in this document makes reference to
syntactic entities that are defined in RFC 822 and not in
this document. A complete formal grammar, then, is obtained
by combining the collected grammar appendix of this document
with that of RFC 822.
The term CRLF, in this document, refers to the sequence of
the two ASCII characters CR (13) and LF (10) which, taken
together, in this order, denote a line break in RFC 822
mail.
The term "character set", wherever it is used in this
document, refers to a coded character set, in the sense of
ISO character set standardization work, and must not be
misinterpreted as meaning "a set of characters."
The term "message", when not further qualified, means either
the (complete or "top-level") message being transferred on a
network, or a message encapsulated in a body of type
"message".
The term "body part", in this document, means one of the
parts of the body of a multipart entity. A body part has a
header and a body, so it makes sense to speak about the body
of a body part.
The term "entity", in this document, means either a message
or a body part. All kinds of entities share the property
that they have a header and a body.
The term "body", when not further qualified, means the body
of an entity, that is the body of either a message or of a
body part.
Note : the previous four definitions are clearly circular.
This is unavoidable, since the overal structure of a MIME
message is indeed recursive.
In this document, all numeric and octet values are given in
decimal notation.
It must be noted that Content-Type values, subtypes, and
parameter names as defined in this document are case-
insensitive. However, parameter values are case-sensitive
unless otherwise specified for the specific parameter.
FORMATTING NOTE: This document has been carefully formatted
for ease of reading. The PostScript version of this
document, in particular, places notes like this one, which
may be skipped by the reader, in a smaller, italicized,
font, and indents it as well. In the text version, only the
indentation is preserved, so if you are reading the text
version of this you might consider using the PostScript
version instead. However, all such notes will be indented
and preceded by "NOTE:" or some similar introduction, even
in the text version.
The primary purpose of these non-essential notes is to
convey information about the rationale of this document, or
to place this document in the proper historical or
evolutionary context. Such information may be skipped by
those who are focused entirely on building a compliant
implementation, but may be of use to those who wish to
understand why this document is written as it is.
For ease of recognition, all BNF definitions have been
placed in a fixed-width font in the PostScript version of
this document.
3 The MIME-Version Header Field Since RFC 822 was published in 1982, there has really been only one format standard for Internet messages, and there has been little perceived need to declare the format standard in use. This document is an independent document that complements RFC 822. Although the extensions in this document have been defined in such a way as to be compatible with RFC 822, there are still circumstances in which it might be desirable for a mail-processing agent to know whether a message was composed with the new standard in mind. Therefore, this document defines a new header field, "MIME- Version", which is to be used to declare the version of the Internet message body format standard in use. Messages composed in accordance with this document MUST include such a header field, with the following verbatim text: MIME-Version: 1.0 The presence of this header field is an assertion that the message has been composed in compliance with this document. Since it is possible that a future document might extend the message format standard again, a formal BNF is given for the content of the MIME-Version field: MIME-Version := text Thus, future format specifiers, which might replace or extend "1.0", are (minimally) constrained by the definition of "text", which appears in RFC 822. Note that the MIME-Version header field is required at the top level of a message. It is not required for each body part of a multipart entity. It is required for the embedded headers of a body of type "message" if and only if the embedded message is itself claimed to be MIME-compliant.
4 The Content-Type Header Field The purpose of the Content-Type field is to describe the data contained in the body fully enough that the receiving user agent can pick an appropriate agent or mechanism to present the data to the user, or otherwise deal with the data in an appropriate manner. HISTORICAL NOTE: The Content-Type header field was first defined in RFC 1049. RFC 1049 Content-types used a simpler and less powerful syntax, but one that is largely compatible with the mechanism given here. The Content-Type header field is used to specify the nature of the data in the body of an entity, by giving type and subtype identifiers, and by providing auxiliary information that may be required for certain types. After the type and subtype names, the remainder of the header field is simply a set of parameters, specified in an attribute/value notation. The set of meaningful parameters differs for the different types. The ordering of parameters is not significant. Among the defined parameters is a "charset" parameter by which the character set used in the body may be declared. Comments are allowed in accordance with RFC 822 rules for structured header fields. In general, the top-level Content-Type is used to declare the general type of data, while the subtype specifies a specific format for that type of data. Thus, a Content-Type of "image/xyz" is enough to tell a user agent that the data is an image, even if the user agent has no knowledge of the specific image format "xyz". Such information can be used, for example, to decide whether or not to show a user the raw data from an unrecognized subtype -- such an action might be reasonable for unrecognized subtypes of text, but not for unrecognized subtypes of image or audio. For this reason, registered subtypes of audio, image, text, and video, should not contain embedded information that is really of a different type. Such compound types should be represented using the "multipart" or "application" types. Parameters are modifiers of the content-subtype, and do not fundamentally affect the requirements of the host system. Although most parameters make sense only with certain content-types, others are "global" in the sense that they might apply to any subtype. For example, the "boundary" parameter makes sense only for the "multipart" content-type, but the "charset" parameter might make sense with several content-types. An initial set of seven Content-Types is defined by this document. This set of top-level names is intended to be substantially complete. It is expected that additions to the larger set of supported types can generally be
accomplished by the creation of new subtypes of these
initial types. In the future, more top-level types may be
defined only by an extension to this standard. If another
primary type is to be used for any reason, it must be given
a name starting with "X-" to indicate its non-standard
status and to avoid a potential conflict with a future
official name.
In the Extended BNF notation of RFC 822, a Content-Type
header field value is defined as follows:
Content-Type := type "/" subtype *[";" parameter]
type := "application" / "audio"
/ "image" / "message"
/ "multipart" / "text"
/ "video" / x-token
x-token := <The two characters "X-" followed, with no
intervening white space, by any token>
subtype := token
parameter := attribute "=" value
attribute := token
value := token / quoted-string
token := 1*<any CHAR except SPACE, CTLs, or tspecials>
tspecials := "(" / ")" / "<" / ">" / "@" ; Must be in
/ "," / ";" / ":" / "\" / <"> ; quoted-string,
/ "/" / "[" / "]" / "?" / "." ; to use within
/ "=" ; parameter values
Note that the definition of "tspecials" is the same as the
RFC 822 definition of "specials" with the addition of the
three characters "/", "?", and "=".
Note also that a subtype specification is MANDATORY. There
are no default subtypes.
The type, subtype, and parameter names are not case
sensitive. For example, TEXT, Text, and TeXt are all
equivalent. Parameter values are normally case sensitive,
but certain parameters are interpreted to be case-
insensitive, depending on the intended use. (For example,
multipart boundaries are case-sensitive, but the "access-
type" for message/External-body is not case-sensitive.)
Beyond this syntax, the only constraint on the definition of
subtype names is the desire that their uses must not
conflict. That is, it would be undesirable to have two
different communities using "Content-Type:
application/foobar" to mean two different things. The
process of defining new content-subtypes, then, is not
intended to be a mechanism for imposing restrictions, but
simply a mechanism for publicizing the usages. There are,
therefore, two acceptable mechanisms for defining new
Content-Type subtypes:
1. Private values (starting with "X-") may be
defined bilaterally between two cooperating
agents without outside registration or
standardization.
2. New standard values must be documented,
registered with, and approved by IANA, as
described in Appendix F. Where intended for
public use, the formats they refer to must
also be defined by a published specification,
and possibly offered for standardization.
The seven standard initial predefined Content-Types are
detailed in the bulk of this document. They are:
text -- textual information. The primary subtype,
"plain", indicates plain (unformatted) text. No
special software is required to get the full
meaning of the text, aside from support for the
indicated character set. Subtypes are to be used
for enriched text in forms where application
software may enhance the appearance of the text,
but such software must not be required in order to
get the general idea of the content. Possible
subtypes thus include any readable word processor
format. A very simple and portable subtype,
richtext, is defined in this document.
multipart -- data consisting of multiple parts of
independent data types. Four initial subtypes
are defined, including the primary "mixed"
subtype, "alternative" for representing the same
data in multiple formats, "parallel" for parts
intended to be viewed simultaneously, and "digest"
for multipart entities in which each part is of
type "message".
message -- an encapsulated message. A body of
Content-Type "message" is itself a fully formatted
RFC 822 conformant message which may contain its
own different Content-Type header field. The
primary subtype is "rfc822". The "partial"
subtype is defined for partial messages, to permit
the fragmented transmission of bodies that are
thought to be too large to be passed through mail
transport facilities. Another subtype,
"External-body", is defined for specifying large
bodies by reference to an external data source.
image -- image data. Image requires a display device
(such as a graphical display, a printer, or a FAX
machine) to view the information. Initial
subtypes are defined for two widely-used image
formats, jpeg and gif.
audio -- audio data, with initial subtype "basic".
Audio requires an audio output device (such as a
speaker or a telephone) to "display" the contents.
video -- video data. Video requires the capability to
display moving images, typically including
specialized hardware and software. The initial
subtype is "mpeg".
application -- some other kind of data, typically
either uninterpreted binary data or information to
be processed by a mail-based application. The
primary subtype, "octet-stream", is to be used in
the case of uninterpreted binary data, in which
case the simplest recommended action is to offer
to write the information into a file for the user.
Two additional subtypes, "ODA" and "PostScript",
are defined for transporting ODA and PostScript
documents in bodies. Other expected uses for
"application" include spreadsheets, data for
mail-based scheduling systems, and languages for
"active" (computational) email. (Note that active
email entails several securityconsiderations,
which are discussed later in this memo,
particularly in the context of
application/PostScript.)
Default RFC 822 messages are typed by this protocol as plain
text in the US-ASCII character set, which can be explicitly
specified as "Content-type: text/plain; charset=us-ascii".
If no Content-Type is specified, either by error or by an
older user agent, this default is assumed. In the presence
of a MIME-Version header field, a receiving User Agent can
also assume that plain US-ASCII text was the sender's
intent. In the absence of a MIME-Version specification,
plain US-ASCII text must still be assumed, but the sender's
intent might have been otherwise.
RATIONALE: In the absence of any Content-Type header field
or MIME-Version header field, it is impossible to be certain
that a message is actually text in the US-ASCII character
set, since it might well be a message that, using the
conventions that predate this document, includes text in
another character set or non-textual data in a manner that
cannot be automatically recognized (e.g., a uuencoded
compressed UNIX tar file). Although there is no fully
acceptable alternative to treating such untyped messages as
"text/plain; charset=us-ascii", implementors should remain
aware that if a message lacks both the MIME-Version and the
Content-Type header fields, it may in practice contain
almost anything.
It should be noted that the list of Content-Type values
given here may be augmented in time, via the mechanisms
described above, and that the set of subtypes is expected to
grow substantially.
When a mail reader encounters mail with an unknown Content-
type value, it should generally treat it as equivalent to
"application/octet-stream", as described later in this
document.
5 The Content-Transfer-Encoding Header Field
Many Content-Types which could usefully be transported via
email are represented, in their "natural" format, as 8-bit
character or binary data. Such data cannot be transmitted
over some transport protocols. For example, RFC 821
restricts mail messages to 7-bit US-ASCII data with 1000
character lines.
It is necessary, therefore, to define a standard mechanism
for re-encoding such data into a 7-bit short-line format.
This document specifies that such encodings will be
indicated by a new "Content-Transfer-Encoding" header field.
The Content-Transfer-Encoding field is used to indicate the
type of transformation that has been used in order to
represent the body in an acceptable manner for transport.
Unlike Content-Types, a proliferation of Content-Transfer-
Encoding values is undesirable and unnecessary. However,
establishing only a single Content-Transfer-Encoding
mechanism does not seem possible. There is a tradeoff
between the desire for a compact and efficient encoding of
largely-binary data and the desire for a readable encoding
of data that is mostly, but not entirely, 7-bit data. For
this reason, at least two encoding mechanisms are necessary:
a "readable" encoding and a "dense" encoding.
The Content-Transfer-Encoding field is designed to specify
an invertible mapping between the "native" representation of
a type of data and a representation that can be readily
exchanged using 7 bit mail transport protocols, such as
those defined by RFC 821 (SMTP). This field has not been
defined by any previous standard. The field's value is a
single token specifying the type of encoding, as enumerated
below. Formally:
Content-Transfer-Encoding := "BASE64" / "QUOTED-PRINTABLE" /
"8BIT" / "7BIT" /
"BINARY" / x-token
These values are not case sensitive. That is, Base64 and
BASE64 and bAsE64 are all equivalent. An encoding type of
7BIT requires that the body is already in a seven-bit mail-
ready representation. This is the default value -- that is,
"Content-Transfer-Encoding: 7BIT" is assumed if the
Content-Transfer-Encoding header field is not present.
The values "8bit", "7bit", and "binary" all imply that NO
encoding has been performed. However, they are potentially
useful as indications of the kind of data contained in the
object, and therefore of the kind of encoding that might
need to be performed for transmission in a given transport
system. "7bit" means that the data is all represented as
short lines of US-ASCII data. "8bit" means that the lines
are short, but there may be non-ASCII characters (octets
with the high-order bit set). "Binary" means that not only
may non-ASCII characters be present, but also that the lines
are not necessarily short enough for SMTP transport.
The difference between "8bit" (or any other conceivable
bit-width token) and the "binary" token is that "binary"
does not require adherence to any limits on line length or
to the SMTP CRLF semantics, while the bit-width tokens do
require such adherence. If the body contains data in any
bit-width other than 7-bit, the appropriate bit-width
Content-Transfer-Encoding token must be used (e.g., "8bit"
for unencoded 8 bit wide data). If the body contains binary
data, the "binary" Content-Transfer-Encoding token must be
used.
NOTE: The distinction between the Content-Transfer-Encoding
values of "binary," "8bit," etc. may seem unimportant, in
that all of them really mean "none" -- that is, there has
been no encoding of the data for transport. However, clear
labeling will be of enormous value to gateways between
future mail transport systems with differing capabilities in
transporting data that do not meet the restrictions of RFC
821 transport.
As of the publication of this document, there are no
standardized Internet transports for which it is legitimate
to include unencoded 8-bit or binary data in mail bodies.
Thus there are no circumstances in which the "8bit" or
"binary" Content-Transfer-Encoding is actually legal on the
Internet. However, in the event that 8-bit or binary mail
transport becomes a reality in Internet mail, or when this
document is used in conjunction with any other 8-bit or
binary-capable transport mechanism, 8-bit or binary bodies
should be labeled as such using this mechanism.
NOTE: The five values defined for the Content-Transfer-
Encoding field imply nothing about the Content-Type other
than the algorithm by which it was encoded or the transport
system requirements if unencoded.
Implementors may, if necessary, define new Content-
Transfer-Encoding values, but must use an x-token, which is
a name prefixed by "X-" to indicate its non-standard status,
e.g., "Content-Transfer-Encoding: x-my-new-encoding".
However, unlike Content-Types and subtypes, the creation of
new Content-Transfer-Encoding values is explicitly and
strongly discouraged, as it seems likely to hinder
interoperability with little potential benefit. Their use
is allowed only as the result of an agreement between
cooperating user agents.
If a Content-Transfer-Encoding header field appears as part
of a message header, it applies to the entire body of that
message. If a Content-Transfer-Encoding header field
appears as part of a body part's headers, it applies only to
the body of that body part. If an entity is of type
"multipart" or "message", the Content-Transfer-Encoding is
not permitted to have any value other than a bit width
(e.g., "7bit", "8bit", etc.) or "binary".
It should be noted that email is character-oriented, so that
the mechanisms described here are mechanisms for encoding
arbitrary byte streams, not bit streams. If a bit stream is
to be encoded via one of these mechanisms, it must first be
converted to an 8-bit byte stream using the network standard
bit order ("big-endian"), in which the earlier bits in a
stream become the higher-order bits in a byte. A bit stream
not ending at an 8-bit boundary must be padded with zeroes.
This document provides a mechanism for noting the addition
of such padding in the case of the application Content-Type,
which has a "padding" parameter.
The encoding mechanisms defined here explicitly encode all
data in ASCII. Thus, for example, suppose an entity has
header fields such as:
Content-Type: text/plain; charset=ISO-8859-1
Content-transfer-encoding: base64
This should be interpreted to mean that the body is a base64
ASCII encoding of data that was originally in ISO-8859-1,
and will be in that character set again after decoding.
The following sections will define the two standard encoding
mechanisms. The definition of new content-transfer-
encodings is explicitly discouraged and should only occur
when absolutely necessary. All content-transfer-encoding
namespace except that beginning with "X-" is explicitly
reserved to the IANA for future use. Private agreements
about content-transfer-encodings are also explicitly
discouraged.
Certain Content-Transfer-Encoding values may only be used on
certain Content-Types. In particular, it is expressly
forbidden to use any encodings other than "7bit", "8bit", or
"binary" with any Content-Type that recursively includes
other Content-Type fields, notably the "multipart" and
"message" Content-Types. All encodings that are desired for
bodies of type multipart or message must be done at the
innermost level, by encoding the actual body that needs to
be encoded.
NOTE ON ENCODING RESTRICTIONS: Though the prohibition
against using content-transfer-encodings on data of type
multipart or message may seem overly restrictive, it is
necessary to prevent nested encodings, in which data are
passed through an encoding algorithm multiple times, and
must be decoded multiple times in order to be properly
viewed. Nested encodings add considerable complexity to
user agents: aside from the obvious efficiency problems
with such multiple encodings, they can obscure the basic
structure of a message. In particular, they can imply that
several decoding operations are necessary simply to find out
what types of objects a message contains. Banning nested
encodings may complicate the job of certain mail gateways,
but this seems less of a problem than the effect of nested
encodings on user agents.
NOTE ON THE RELATIONSHIP BETWEEN CONTENT-TYPE AND CONTENT-
TRANSFER-ENCODING: It may seem that the Content-Transfer-
Encoding could be inferred from the characteristics of the
Content-Type that is to be encoded, or, at the very least,
that certain Content-Transfer-Encodings could be mandated
for use with specific Content-Types. There are several
reasons why this is not the case. First, given the varying
types of transports used for mail, some encodings may be
appropriate for some Content-Type/transport combinations and
not for others. (For example, in an 8-bit transport, no
encoding would be required for text in certain character
sets, while such encodings are clearly required for 7-bit
SMTP.) Second, certain Content-Types may require different
types of transfer encoding under different circumstances.
For example, many PostScript bodies might consist entirely
of short lines of 7-bit data and hence require little or no
encoding. Other PostScript bodies (especially those using
Level 2 PostScript's binary encoding mechanism) may only be
reasonably represented using a binary transport encoding.
Finally, since Content-Type is intended to be an open-ended
specification mechanism, strict specification of an
association between Content-Types and encodings effectively
couples the specification of an application protocol with a
specific lower-level transport. This is not desirable since
the developers of a Content-Type should not have to be aware
of all the transports in use and what their limitations are.
NOTE ON TRANSLATING ENCODINGS: The quoted-printable and
base64 encodings are designed so that conversion between
them is possible. The only issue that arises in such a
conversion is the handling of line breaks. When converting
from quoted-printable to base64 a line break must be
converted into a CRLF sequence. Similarly, a CRLF sequence
in base64 data should be converted to a quoted-printable
line break, but ONLY when converting text data.
NOTE ON CANONICAL ENCODING MODEL: There was some
confusion, in earlier drafts of this memo, regarding the
model for when email data was to be converted to canonical
form and encoded, and in particular how this process would
affect the treatment of CRLFs, given that the representation
of newlines varies greatly from system to system. For this
reason, a canonical model for encoding is presented as
Appendix H.
5.1 Quoted-Printable Content-Transfer-Encoding
The Quoted-Printable encoding is intended to represent data
that largely consists of octets that correspond to printable
characters in the ASCII character set. It encodes the data
in such a way that the resulting octets are unlikely to be
modified by mail transport. If the data being encoded are
mostly ASCII text, the encoded form of the data remains
largely recognizable by humans. A body which is entirely
ASCII may also be encoded in Quoted-Printable to ensure the
integrity of the data should the message pass through a
character-translating, and/or line-wrapping gateway.
In this encoding, octets are to be represented as determined
by the following rules:
Rule #1: (General 8-bit representation) Any octet,
except those indicating a line break according to the
newline convention of the canonical form of the data
being encoded, may be represented by an "=" followed by
a two digit hexadecimal representation of the octet's
value. The digits of the hexadecimal alphabet, for this
purpose, are "0123456789ABCDEF". Uppercase letters must
be
used when sending hexadecimal data, though a robust
implementation may choose to recognize lowercase
letters on receipt. Thus, for example, the value 12
(ASCII form feed) can be represented by "=0C", and the
value 61 (ASCII EQUAL SIGN) can be represented by
"=3D". Except when the following rules allow an
alternative encoding, this rule is mandatory.
Rule #2: (Literal representation) Octets with decimal
values of 33 through 60 inclusive, and 62 through 126,
inclusive, MAY be represented as the ASCII characters
which correspond to those octets (EXCLAMATION POINT
through LESS THAN, and GREATER THAN through TILDE,
respectively).
Rule #3: (White Space): Octets with values of 9 and 32
MAY be represented as ASCII TAB (HT) and SPACE
characters, respectively, but MUST NOT be so
represented at the end of an encoded line. Any TAB (HT)
or SPACE characters on an encoded line MUST thus be
followed on that line by a printable character. In
particular, an "=" at the end of an encoded line,
indicating a soft line break (see rule #5) may follow
one or more TAB (HT) or SPACE characters. It follows
that an octet with value 9 or 32 appearing at the end
of an encoded line must be represented according to
Rule #1. This rule is necessary because some MTAs
(Message Transport Agents, programs which transport
messages from one user to another, or perform a part of
such transfers) are known to pad lines of text with
SPACEs, and others are known to remove "white space"
characters from the end of a line. Therefore, when
decoding a Quoted-Printable body, any trailing white
space on a line must be deleted, as it will necessarily
have been added by intermediate transport agents.
Rule #4 (Line Breaks): A line break in a text body
part, independent of what its representation is
following the canonical representation of the data
being encoded, must be represented by a (RFC 822) line
break, which is a CRLF sequence, in the Quoted-
Printable encoding. If isolated CRs and LFs, or LF CR
and CR LF sequences are allowed to appear in binary
data according to the canonical form, they must be
represented using the "=0D", "=0A", "=0A=0D" and
"=0D=0A" notations respectively.
Note that many implementation may elect to encode the
local representation of various content types directly.
In particular, this may apply to plain text material on
systems that use newline conventions other than CRLF
delimiters. Such an implementation is permissible, but
the generation of line breaks must be generalized to
account for the case where alternate representations of
newline sequences are used.
Rule #5 (Soft Line Breaks): The Quoted-Printable
encoding REQUIRES that encoded lines be no more than 76
characters long. If longer lines are to be encoded with
the Quoted-Printable encoding, 'soft' line breaks must
be used. An equal sign as the last character on a
encoded line indicates such a non-significant ('soft')
line break in the encoded text. Thus if the "raw" form
of the line is a single unencoded line that says:
Now's the time for all folk to come to the aid of
their country.
This can be represented, in the Quoted-Printable
encoding, as
Now's the time =
for all folk to come=
to the aid of their country.
This provides a mechanism with which long lines are
encoded in such a way as to be restored by the user
agent. The 76 character limit does not count the
trailing CRLF, but counts all other characters,
including any equal signs.
Since the hyphen character ("-") is represented as itself in
the Quoted-Printable encoding, care must be taken, when
encapsulating a quoted-printable encoded body in a multipart
entity, to ensure that the encapsulation boundary does not
appear anywhere in the encoded body. (A good strategy is to
choose a boundary that includes a character sequence such as
"=_" which can never appear in a quoted-printable body. See
the definition of multipart messages later in this
document.)
NOTE: The quoted-printable encoding represents something of
a compromise between readability and reliability in
transport. Bodies encoded with the quoted-printable
encoding will work reliably over most mail gateways, but may
not work perfectly over a few gateways, notably those
involving translation into EBCDIC. (In theory, an EBCDIC
gateway could decode a quoted-printable body and re-encode
it using base64, but such gateways do not yet exist.) A
higher level of confidence is offered by the base64
Content-Transfer-Encoding. A way to get reasonably reliable
transport through EBCDIC gateways is to also quote the ASCII
characters
!"#$@[\]^`{|}~
according to rule #1. See Appendix B for more information.
Because quoted-printable data is generally assumed to be
line-oriented, it is to be expected that the breaks between
the lines of quoted printable data may be altered in
transport, in the same manner that plain text mail has
always been altered in Internet mail when passing between
systems with differing newline conventions. If such
alterations are likely to constitute a corruption of the
data, it is probably more sensible to use the base64
encoding rather than the quoted-printable encoding.
5.2 Base64 Content-Transfer-Encoding The Base64 Content-Transfer-Encoding is designed to represent arbitrary sequences of octets in a form that is not humanly readable. The encoding and decoding algorithms are simple, but the encoded data are consistently only about 33 percent larger than the unencoded data. This encoding is based on the one used in Privacy Enhanced Mail applications, as defined in RFC 1113. The base64 encoding is adapted from RFC 1113, with one change: base64 eliminates the "*" mechanism for embedded clear text. A 65-character subset of US-ASCII is used, enabling 6 bits to be represented per printable character. (The extra 65th character, "=", is used to signify a special processing function.) NOTE: This subset has the important property that it is represented identically in all versions of ISO 646, including US ASCII, and all characters in the subset are also represented identically in all versions of EBCDIC. Other popular encodings, such as the encoding used by the UUENCODE utility and the base85 encoding specified as part of Level 2 PostScript, do not share these properties, and thus do not fulfill the portability requirements a binary transport encoding for mail must meet. The encoding process represents 24-bit groups of input bits as output strings of 4 encoded characters. Proceeding from left to right, a 24-bit input group is formed by concatenating 3 8-bit input groups. These 24 bits are then treated as 4 concatenated 6-bit groups, each of which is translated into a single digit in the base64 alphabet. When encoding a bit stream via the base64 encoding, the bit stream must be presumed to be ordered with the most- significant-bit first. That is, the first bit in the stream will be the high-order bit in the first byte, and the eighth bit will be the low-order bit in the first byte, and so on. Each 6-bit group is used as an index into an array of 64 printable characters. The character referenced by the index is placed in the output string. These characters, identified in Table 1, below, are selected so as to be universally representable, and the set excludes characters with particular significance to SMTP (e.g., ".", "CR", "LF") and to the encapsulation boundaries defined in this document (e.g., "-").
Table 1: The Base64 Alphabet
Value Encoding Value Encoding Value Encoding Value
Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y
The output stream (encoded bytes) must be represented in
lines of no more than 76 characters each. All line breaks
or other characters not found in Table 1 must be ignored by
decoding software. In base64 data, characters other than
those in Table 1, line breaks, and other white space
probably indicate a transmission error, about which a
warning message or even a message rejection might be
appropriate under some circumstances.
Special processing is performed if fewer than 24 bits are
available at the end of the data being encoded. A full
encoding quantum is always completed at the end of a body.
When fewer than 24 input bits are available in an input
group, zero bits are added (on the right) to form an
integral number of 6-bit groups. Output character positions
which are not required to represent actual input data are
set to the character "=". Since all base64 input is an
integral number of octets, only the following cases can
arise: (1) the final quantum of encoding input is an
integral multiple of 24 bits; here, the final unit of
encoded output will be an integral multiple of 4 characters
with no "=" padding, (2) the final quantum of encoding input
is exactly 8 bits; here, the final unit of encoded output
will be two characters followed by two "=" padding
characters, or (3) the final quantum of encoding input is
exactly 16 bits; here, the final unit of encoded output will
be three characters followed by one "=" padding character.
Care must be taken to use the proper octets for line breaks
if base64 encoding is applied directly to text material that
has not been converted to canonical form. In particular,
text line breaks should be converted into CRLF sequences
prior to base64 encoding. The important thing to note is
that this may be done directly by the encoder rather than in
a prior canonicalization step in some implementations.
NOTE: There is no need to worry about quoting apparent
encapsulation boundaries within base64-encoded parts of
multipart entities because no hyphen characters are used in
the base64 encoding.
6 Additional Optional Content- Header Fields
6.1 Optional Content-ID Header Field
In constructing a high-level user agent, it may be desirable
to allow one body to make reference to another.
Accordingly, bodies may be labeled using the "Content-ID"
header field, which is syntactically identical to the
"Message-ID" header field:
Content-ID := msg-id
Like the Message-ID values, Content-ID values must be
generated to be as unique as possible.
6.2 Optional Content-Description Header Field
The ability to associate some descriptive information with a
given body is often desirable. For example, it may be useful
to mark an "image" body as "a picture of the Space Shuttle
Endeavor." Such text may be placed in the Content-
Description header field.
Content-Description := *text
The description is presumed to be given in the US-ASCII
character set, although the mechanism specified in [RFC-
1342] may be used for non-US-ASCII Content-Description
values.
(next page on part 2)
